Rounding to varying precision

test.js

n = [
    [1,'1'],
    [6,'6'],
    [9,'9'],
    [10,'10'],
    [11,'11'],
    [19,'19'],
    [90,'90'],
    [99,'99'],
    [100,'100'],
    [101,'101'],
    [150,'150'],
    [403,'403'],
    [708,'708'],
    [992,'992'],
    [999,'999'],
    [1000,'1.00k'],
    [1001,'1.00k'],
    [1472,'1.47k'],
    [1920,'1.92k'],
    [1999,'2.00k'],
    [2000,'2.00k'],
    [4032,'4.03k'],
    [9813,'9.81k'],
    [9999,'10.0k'],
    [1e4, '10.0k'],
    [10010,'10.0k'],
    [60712,'60.7k'],
    [98712,'98.7k'],
    [99999,'100k'],
    [1e5,'100k'],
    [1e5+10,'100k'],
    [4e5+10,'400k'],
    [9e5,'900k'],
    [999999,'1.00M'],
    [1e6,'1.00M'],
    [1234567,'1.23M'],
    [12345678,'12.3M'],
    [123456789,'123M'],
]

function siRound(x) {
    if (x<1e3) return x+'';
    let digits = Math.round(Math.log10(x)*100)/100 | 0
    let tier = digits/3 | 0
    let scale = 10**(tier*3)
    return (x/scale).toFixed(2-digits%3) + (["","k","M","G"])[tier]
}

n.forEach( ([x,expected]) => console.log(x, expected, siRound(x)))